For sonic decoupling between the diaphragm cup (ultrasonic generator) and the sensor housing or the sensor receptacle in the bumper, a silicone ring is usually used as a decoupling element. An assemblage between the diaphragm cup and the sensor housing is intended both to avoid parasitic sonic radiation as well as to dampen structure-borne noise impressed by the sensor diaphragm. Ultrasonic sensors having decoupling elements are discussed in patent documents US 2009/0196121 A1, WO 2007/144047, DE 102 37 720 A1, DE 102 37 721 A1 and DE 10 2007 043 500 A1.
It is believed that it may not be possible to ensure that no gap exists between the decoupling element and the diaphragm cup or the housing through which sound enters in the axial direction and results in malfunctions. In particular, a visual check of the area between the decoupling element and the housing is no longer possible after installation. This also applies especially in the case in which, on the viewing side, the decoupling element ends flush with the diaphragm cup. Thus, the situation may arise, in which the sound of the diaphragm cup exits through the gap and results in a sound propagation into undesired spaces such as the rear space of the bumper. Furthermore, there exists the possibility that the structure-borne sound is not damped and reinforces the effect of the unmotivated sonic radiation. Non-planar surfaces may also result in gaps.